(1) Field of the Invention
The present invention generally relates to methods for welding metal alloys. More particularly, this invention relates to a method of electron beam (EB) welding a joint between articles formed of dissimilar metal alloys, as well as to a method of electron beam welding an arcuate contact surface interface between articles.
(2) Description of the Related Art
Various high-temperature alloys are widely used to form hot section components of turbines, including turbine vanes (nozzles) and blades (buckets) of gas and steam turbines. Circumstances exist where such components are preferably or necessarily fabricated by welding. For example, components having complex configurations, such as steam turbine nozzle assemblies (boxes), can be more readily fabricated by welding castings together. Various welding techniques have been developed for this purpose. Tungsten inert gas (TIG) and plasma transferred arc (PTA) techniques are widely used in manual welding operations. For more demanding applications, such as weld joints having high aspect ratios, laser beam and electron beam welding processes have been developed.
As known in the art, electron beam welding involves directing a beam of high-energy electrons on a joint between articles held in a vacuum. Electron beam welding techniques are particularly well suited for producing weld joints having high aspect ratios, as electron beam welding yields the deepest penetrations of any beam process, e.g., on the order of about four inches (about ten centimeters) and greater, with very high aspect ratios of about ten to fifty being readily achieved. However, when electron beam welding articles formed of dissimilar metals and requiring a relatively deep weld joint, a frequently encountered problem is that the beam will “hook” at the bottom of the weld, curving over into one of the articles and missing the joint, yielding a lack-of-fusion (LOF) defect. As an example, a hooked weld joint may result when welding turbine stator vane assemblies whose vanes are welded to inner and outer bands. Such a situation is schematically represented in FIG. 1, in which an electron beam gun 22 is represented as projecting an electron beam 20 onto a contact interface 14 between two components 10 and 12 formed of dissimilar metals. The weld joint 16 formed by the electron beam 20 can be seen to curve into the lefthand component 10, forming what can be termed a hook 24. As a result of the hook 24, the resulting weld joint 16 is incomplete, yielding a lack-of-fusion defect 18 at the extremity of the contact interface 14 opposite the gun 22. The cause of this hook 24 has been debated. Possible causes include a thermal electromotive force (emf) effect, or an electronegativity difference in the dissimilar metals.
Intuitive approaches to correcting this problem, such as biasing the articles 10 and 12 under the beam 20 or orienting the contact interface 14 at an angle to the beam 20, have proven ineffective because as the interface 14 moves, so does the electron beam 20. Relatively minor stray magnetic fields that may be present as a result of using magnetic fixtures or residual magnetism in machined parts are also known to cause significant beam movement. As a result, such parts are frequently checked with a gauss meter and degaussed if required prior to welding. However, such measures are insufficient to eliminate the beam hook 24 represented in FIG. 1.
In view of the above, it would be desirable if the hooking of an electron beam when welding dissimilar metals could be eliminated, allowing for electron beam welding of a greater variety of components that require deep weld joints with high aspect ratios.